JavaScript Data Types

String, Number, Boolean, Array, Object.

JavaScript Data Types

JavaScript variables can hold many data types: numbers, strings, arrays, objects and more:

var length = 16;                               // Number
var lastName = "Johnson";                      // String
var cars = ["Saab""Volvo""BMW"];           // Array
var x = {firstName:"John", lastName:"Doe"};    // Object

The Concept of Data Types

In programming, data types is an important concept.

To be able to operate on variables, it is important to know something about the type.

Without data types, a computer cannot safely solve this:

var x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will it produce a result? Will it produce an error?

JavaScript will treat the example above as:

var x = "16" + "Volvo";
NoteIf the second operand is a string, JavaScript will also treat the first operand as a string. 


var x = 16 + "Volvo";

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JavaScript evaluates expressions from left to right. Different sequences can produce different results:


var x = 16 + 4 + "Volvo";



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var x = "Volvo" + 16 + 4;



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In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".

In the second example, since the first operand is a string, all operands are treated as strings.

JavaScript Has Dynamic Types

JavaScript has dynamic types. This means that the same variable can be used as different types:


var x;               // Now x is undefined
var x = 5;           // Now x is a Number
var x = "John";      // Now x is a String

JavaScript Strings

A string (or a text string) is a series of characters like "John Doe".

Strings are written with quotes. You can use single or double quotes:


var carName = "Volvo XC60";   // Using double quotes
var carName = 'Volvo XC60';   // Using single quotes

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:


var answer = "It's alright";             // Single quote inside double quotes
var answer = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer = 'He is called "Johnny"';    // Double quotes inside single quotes

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You will learn more about strings later in this tutorial.

JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:


var x1 = 34.00;     // Written with decimals
var x2 = 34;        // Written without decimals

Extra large or extra small numbers can be written with scientific (exponential) notation:


var y = 123e5;      // 12300000
var z = 123e-5;     // 0.00123

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You will learn more about numbers later in this tutorial.

JavaScript Booleans

Booleans can only have two values: true or false.


var x = true;
var y = false;

Booleans are often used in conditional testing.

You will learn more about conditional testing later in this tutorial.

JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars, containing three items (car names):


var cars = ["Saab""Volvo""BMW"];

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Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn more about arrays later in this tutorial.

JavaScript Objects

JavaScript objects are written with curly braces.

Object properties are written as name:value pairs, separated by commas.


var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

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The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn more about objects later in this tutorial.

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable:


typeof "John"                // Returns string 
typeof 3.14                  // Returns number
typeof false                 // Returns boolean
typeof [1,2,3,4]             // Returns object
typeof {name:'John', age:34} // Returns object

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NoteIn JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object. 


In JavaScript, a variable without a value, has the value undefined. The typeof is also undefined.


var person;                  // Value is undefined, type is undefined

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Any variable can be emptied, by setting the value to undefined. The type will also be undefined.


person = undefined;          // Value is undefined, type is undefined

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Empty Values

An empty value has nothing to do with undefined.

An empty string variable has both a value and a type.


var car = "";                // The value is "", the typeof is string

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In JavaScript null is "nothing". It is supposed to be something that doesn't exist.

Unfortunately, in JavaScript, the data type of null is an object.

NoteYou can consider it a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:


var person = null;           // Value is null, but type is still an object

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You can also empty an object by setting it to undefined:


var person = undefined;     // Value is undefined, type is undefined

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Difference Between Undefined and Null

typeof undefined             // undefined
typeof null                  // object
null === undefined           // false
null == undefined            // true

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Posted by linuxism